The present invention relates generally to Nuclear Magnetic Resonance (NMR) imaging test devices. More particularly, the invention relates to a physiological phantom for testing image quality, system performance and calibration characteristics of an NMR imaging (MRI) system. The phantom permits the system user to perform the entire series of image quality, system performance, and calibration tests with a single phantom. It also permits the user to perform these tests without repositioning the phantom or reconfiguring the phantom after repositioning.
Well known NMR techniques acquire spectroscopic and imaging information about the internal anatomical features of a subject, such as a human. A system operator analyzes this information to determine such tissue-related parameters as nuclear spin distribution, spin-lattice (T1), and/or spin-spin (T2) relaxation constants believed to be of medical diagnostic value in determining the state of health of tissue in the region examined. In the course of an examination, the system operator positions the patient region of interest in a substantially uniform, static (B.sub.o) magnetic field produced by one of several known means, most commonly superconductive magnets. The MRI system operator collects spectroscopic and imaging data by subjecting the region of interest to pulse sequences comprised of magnetic field gradients and radio frequency (RF) pulses. Separate coil assemblies positioned in the polarizing magnetic field generate the magnetic gradient and RF fields. These fields have generally cylindrical configurations to accommodate the patient region to be studied. The resonant frequency of the RF coil is based on the strength of the static magnetic field and the type of nucleus (e.g., hydrogen, phosphorus, etc.) to be examined.
Known phantoms generally comprise test objects constructed to simulate structures and conditions encountered in actual use. The phantom can be made to simulate various types of tissue and, ideally, should simulate such tissue in its environment. That is, an organ under test, such as a heart or a liver, is generally surrounded in the human body by other NMR active tissue. So, a phantom that provides for testing a particular performance parameter, such as geometric distortion or slice thickness, should include a test element that is also generally surrounded by NMR active and radio frequency (RF) conductive material to more accurately portray NMR system performance. Further, such a phantom should provide test elements at a plurality of locations throughout the region of interest to determine system performance at various points.
Such a phantom can be used as a substitute test object in operator training, as a calibration device to determine the level of equipment performance, and as a standard by which to judge and predict image and spectra quality. In some cases, system operators may wish to determine the degree of equipment operability by daily calibration procedures. Therefore, use of the phantom must allow evaluation of multiple image-quality parameters with relative ease, and a minimum expenditure of operator time and effort. Accordingly, such a phantom should minimize such factors as scan time to acquire the test data, phantom set-up time, and cost. Conversely, such a phantom should maximize such factors as reliability, repeatability and simplicity.
Those skilled in the MRI art recognize that a test subject within an MRI system "loads" the RF coil. That is, the test subject provides a path of conduction for the RF energy produced by the RF coil. The RF load is related to the coil quality factor Q, the coil resonant frequency, the RF field distribution, and the impedance of the coil when the test subject is placed inside. Thus, for example, an "unloaded" RF coil may have a Q of approximately 250, while a coil with a 75 kg person positioned therein may have its Q lowered to 65. The load to the coil determines the amount of power required from the RF power amplifier necessary to perform the NMR experiment, and determines the level of noise which is included in the received NMR signal used to construct an image. If the load on the RF coil is too low, the RF system will not be stressed adequately, and the noise in the image will not be representative of that found in an anatomical image. Thus, a phantom should "load" the RF coil like the anticipated subject will load the coil.
This means, not only should the phantom present an overall load to the RF coil that approximates that of the anticipated subject, but this loading of the RF coil should be distributed throughout the entire region of interest. That is, the NMR active and RF conductive material within the phantom should be distributed as homogeneously as possible throughout the region of interest. Otherwise, images of the phantom will not accurately predict images of the physiological region of interest.
Further, one tissue type can effect the image presented by a nearby second tissue type. Similarly, a phantom will present an inaccurate image if the tissue simulating material is completely surrounded by a nonconductive, nonabsorptive material, such as acrylic or air. Consequently, a phantom should present tissue equivalent material in an environment like that of the anticipated test subject. For example, if an operator wishes to contrast gray matter and white matter in a patient's head, then the phantom should offer material that simulates one tissue type within an environment of the other tissue type.